This patent application is pursuant to Disclosure Document No. 328153 with PTO's acknowledgement of receipt Mar. 29, 1993.
1. Field of Invention
This invention relates to the field of fracture management in Orthopaedic Surgery where it is necessary to drill transverse holes through bones in alignment with distal holes in implants called intramedullary nails. These holes are required for placement of locking screws that fix the broken bone segments while healing occurs. The procedure for aligning and guiding of the drill is generally referred to as "distal targeting." Accurate location of the drilled holes is essential for success of the surgical technique. Avoidance of exposure to x-rays is required for the surgeon's safety. And speed in completing the procedure is important for operating room efficiency.
2. Description of Prior Art
All distal targeting techniques employ a bushing (cylindrical sleeve) that guides the drill, but the various approaches differ in the means of aligning the guide bushing and keeping it in place:
One method is to support the bushing in a fixture that is mechanically attached to the nail. This works well in the targeting of proximal holes that are close to a slot in the end of the nail that may be used for registering the position of the bushing. It works poorly, however, when the greater length of fixture required to support the bushing over the distal holes magnifies the positional error due to manufacturing tolerances on the slot in the nail and the mating lug on the bushing support fixture. Accuracy in positioning the drill bushing over the distal holes is also adversely affected by bending and/or twisting of the nail as it is driven into the medullary canal.
The bushing can be positioned over the hole with the aid of a C-arm (a portable fluoroscopic device that allows directional control of the x-ray beam) and locked in place to complete the drilling. Disadvantages of using the C-arm are the surgeon's risk of x-ray exposure and the surgeon's time and experience required for accurate alignment by trial exposures and repositionings to make the image of holes in the nail appear as perfect circles. Also, the mechanical structures rigid enough to hold the bushing in place while drilling are unwieldy.
Another distal targeting technique is the "freehand method," that also involves the use of C-arms to view the field of operation. In an ideal procedure, the x-ray beam is first positioned along the axis of the transverse hole and the point of a drill or trocar is placed onto the surface of the bone at the desired point of entry. It is then rotated about the point of contact with the bone, into alignment with the beam, as determined by the appearance of truly circular images of the drill or trocar or of reference rings on a drill guide. The drill or trocar is held in this orientation while driven through the bone by a rotating power unit or mallet. In practice, there are several problems with the freehand procedure.
First, it is difficult to determine when the x-ray beam is accurately aligned with a nail hole. C-arms are bulky machines that are hard to position precisely, they must be turned on and off repeatedly during positioning, and alignment depends on the surgeon's judgement that truly circular images of holes are achieved. And after the C-arm is aligned as well as possible, the alignment can be disrupted by bumping into the C-arm or by a shift of a patient's position on the operating table. A second problem is that after the drill or trocar has been positioned and the x-ray has been turned off; the surgeon must hold that position while drilling or driving the trocar through the bone--without the assistance of the fluoroscopic image. The freehand method, therefore, depends heavily on the experience and steady hand of the surgeon. A third problem with the method is that, due to the manual difficulty, the procedure is often lengthy, and prolonging the duration of surgical procedures is detrimental to patients and decreases the cost effectiveness of operating rooms. A fourth problem is the hazard of radiation exposure while trying to work with hands close to the x-ray beam. This is difficult to quantify, but, in attempting to minimize their exposure risk, surgeons are tempted to rush the procedure or work at distances that make accurate alignment very difficult.
To alleviate the problem of holding steady for long periods of time while the fluoroscopic display is turned off, some orthopaedic equipment manufacturers provide "radiolucent drills." These are, in effect, powered radiolucent drill guides which allow drilling to be performed with the x-ray beam on. This would seem to improve the freehand method, but radiolucent drills are "right angled" devices that must be held from an awkward side position, in order to keep the surgeon's hands out of the x-ray beam. It is difficult to maintain alignment while applying adequate force from the side position. And leaving the x-ray on fur longer times increases the patient's radiation exposure and increases the surgeon's risk of exposure.
In the apparatus of U.S. Pat. No. 4,621,628 to Brudermann, Nov. 11, 1986, a toroidally shaped magnet is placed on the surface of the skin in the vicinity of a transverse hole in an intramedullary nail, into which has been placed a probe that positions Hall effect sensors in the nail and on the axis line of the transverse hole. Signals derived from the sensors are to indicate deviations of the alignment of the axes of the toroidal magnet and the transverse hole. It is assumed that the magnetic field lines in the vicinity of the axis of the toroidal magnet are linear and parallel, and that, therefore, a zero point indication from the Hall sensors indicates that the axis of the toroidal magnet at the surface of the body is on the axis of the transverse hole.
If the magnet is small and the sensors are far away in relation to its size, then at any position in the vicinity of the axis of the transverse hole, there is an angular orientation of the magnet that will produce a zero output of the Hall sensors. Conversely, if the magnet is large in relation to the distance from the sensors, and the field lines are approximately parallel, then there are any number of translational positions with the same angular orientation that will produce nearly zero output of the Hall sensors. For this reason, positioning of the toroidal magnet so as to zero the output of the Hall sensors will not with sufficient accuracy determine the unique position on the surface of the body that is on the axis of the hole, as is required for successful operation of the apparatus. Accordingly, the apparatus uses a second, pin-shaped magnet inserted through the toroidal magnet, and positioned it in a similar manner. This is intended to determine a second search point, which in conjuction with the search point on the surface of the body, would define the axis of the transverse hole. For reasons stated above in connection with the placement of the toroidal magnet, placement of the second magnet will also be adversely affected. This would be so, even if the previously placed toroidal magnet were removed from the scene, so it was not simultaneously contributing to the magnetic field at the sensors. However, since the toroidal magnet is in fact present, and because the magnetic field at the sensors will be the vector sum of the fields generated by both magnets, placement of the second magnet is not an independent determination of a second search point on the axis of the transverse hole. To complete the procedure, the pin-shaped magnet is replaced in the drilling jig by a drill bushing, the Hall sensors are removed from the nail, and the bone is drilled through both the near and far cortices. From the point in time that the Hall sensors are removed from the nail, accuracy depends on the surgeon's ability to hold the drilling jig in the selected position, without the assistance of the display device. The technique, therefore, depends strongly on the surgeon's steady hand and experience, as does the freehand method employing an x-ray machine.
In the drill guide of European patent application number 92306267.3 by Stryker Corporation a coil mounted within the nail generates a magnetic field along the axis of the transverse, and eight sensing coils determine when the guide tube is aligned with the transverse hole. The axis of the coil within the nail is coincident with the axis of the hole. So either the drill must pass through the center of the coil and the coil must be removed before the screw is placed, or the coil must be removed before passing the drill through the nail to the far cortex of bone, in order to avoid hitting the coil with the drill bit.
The two methods referenced above have an important disadvantage over the C-arm freehand method, in that usefulness of the display is lost once Brudermann's second magnet and then the Hall sensors are removed, or once Stryker's coil within the nail is removed. Their display devices cannot be used even to re-check alignment as drilling progresses, as is possible in the C-arm freehand method..